BRUSSELS, Belgium (CNN) -- NATO announced Saturday that it will move to the second phase of its air campaign against Yugoslavia, with a new emphasis on low-altitude attacks on tanks and heavy weapons on the ground in Kosovo.

Pentagon spokesman Ken Bacon later confirmed that the plane had gone down -- the first NATO plane lost in the four-day bombing campaign, according to NATO. The pilot was rescued by an American search-and-rescue team, he said.

Bacon said U.S. officials do not know yet whether the plane was shot down or crashed for some other reason.

Sources close to the Yugoslav government claimed other NATO aircraft had been shot down and two pilots were captured, but there was no independent confirmation of those claims.

In an interview with CNN, NATO spokesman Jamie Shea said the purpose of the second phase of the airstrikes would be "to go after the heavy weapons and the command posts and the headquarters which are used to sustain that violent campaign" against ethnic Albanians in Kosovo.

NATO officials said that while Yugoslav troops inside their barracks would not be targeted, troops in the field would be subject to attack.

British Foreign Secretary Robin Cook told CNN's Christiane Amanpour that NATO's strategy in the second phase of the air campaign is "to make sure that we do make it very difficult for the Yugoslav army to support the repression in Kosovo and that we increase the cost of (Yugoslav President Slobodan) Milosevic of doing that."

In Washington, White House and State Department officials said the decision to go to phase two had been part of NATO's plan from the beginning of the air campaign and was not a reaction to reports of stepped-up attacks on ethnic Albanian civilians in Kosovo.

NATO Secretary-General Javier Solana ordered the move to the second phase after consultations with ambassadors from the 19 NATO countries.

A NATO military source told CNN that commanders believe that the first three days of airstrikes damaged Yugoslav air defense to a "sufficient level" to go to the second phase of the campaign with "minimal risk" to NATO personnel.

The attacks called for in phase two -- which could present more danger to NATO pilots, who will be flying lower and more slowly -- will be carried out primarily in Kosovo and areas just to the north of the province, Shea said. But he said attacks on air defenses and other military facilities would continue in other parts of Yugoslavia.

On Saturday, NATO launched a fourth day of air attacks in Yugoslavia. A series of loud explosions could be heard in Belgrade, though the detonations appeared to be well outside the city.

The location where Yugoslav officials said the F-117 was shot down is not in the area where NATO officials said the second phase of the air operation would be concentrated. Bacon said its mission was planned before NATO made its decision to change the emphasis of the air campaign.

On Saturday, thousands more Albanian refugees left Kosovo and poured into neighboring Albania and Macedonia, telling of forced roundups and expulsions. Independent confirmation of those reports was difficult because most international journalists and observers have been expelled from Kosovo by the Serbian government.

A village in Kosovo just north of Macedonia was on fire Saturday, and residents who fled across the border told CNN's Chris Burns that police had ordered them to leave their homes within two hours or be killed.

Cook told CNN that NATO officials have received reports of "severe fighting" inside Kosovo, and he warned Yugoslav officials that they would be held responsible for attacks on civilians.

"Those who carry out those acts should know that what they're doing is committing a war crime. They will be held accountable for it, and it will not just be the field commanders," Cook said. "Any political leaders we find who have ordered that type of behavior will also be held to account."

The separatist Kosovo Liberation Army, the target of the Yugoslav crackdown in Kosovo, called on NATO Saturday to send ground troops.

"We welcome the (NATO) strikes, but we demand that NATO ground troops come to Kosovo as quickly as possible," a KLA spokesman said at a news conference at NATO headquarters in Brussels.

But NATO leaders, including U.S. President Bill Clinton, have made it clear there are no plans to introduce ground troops in a combat role. In his remarks to CNN, Cook reiterated that stand.

"We are not capable, we do not have the capacity in theater, and we do not have the intention of committing ground troops. To assemble the expeditionary force to fight its way in would in itself probably be a matter of several months, and we are not intending to undertake that exercise," he said.

In Washington, Bacon released a statement saying the United States "has no intention of sending ground troops to fight in Kosovo, and the Department of Defense is not doing any planning that would enable such a deployment."

On state television Saturday, Milosevic lambasted the NATO attacks and called for international support.

"It is the duty of all free countries to stand up to the military despotism of NATO, led by the United States, which is destroying the United Nations system and represents the most serious threat to international peace and security since World War II," Milosevic said.

He had met with the Ukrainian defense and foreign ministers, who came to Belgrade to try to mediate Yugoslavia's standoff with the West.

"As an attacked country, Yugoslavia should get all necessary assistance," added the defiant Yugoslav leader, alluding to traditional allies of Serbs, including Russia and Ukraine.

But President Milo Djukanovic of Montenegro, one of the two republics that make up the Yugoslav federation, called on Milosevic Saturday to resume peace talks with the Kosovar Albanians.

After Djukanovic met with his Cabinet in the Montenegrin capital, Podgorica, the leadership issued a statement that "conflict with the world, which has kept Yugoslavia isolated, is no policy for the future of our people and our state."

Djukanovic has been a longtime critic of Milosevic, but he also called on NATO to end the air offensive. Although Montenegro declared neutrality in the dispute, military targets in Montenegro have been bombed.

In his weekly radio address to the American people Saturday, Clinton said the increased Serb offensive in Kosovo was even more reason for NATO allies to "stay the course" and continue the air attacks on Yugoslav targets.

Clinton spoke by telephone Saturday with the leaders of Britain, France, Germany and Italy to discuss events in Yugoslavia.

NATO officials say the bombings will continue until Milosevic accepts a proposed peace plan for Kosovo, or until NATO considers the Yugoslav armed forces so reduced that they will not be able to carry on attacks in Kosovo.